A number of improvements in the technique of artificial insemination of animals have been made in recent years to improve the effectiveness and efficiency of results. An initial problem was the fact that semen could not be efficiently stored for any appreciable length of time. This hurdle was overcome by utilizing a process of freezing the semen at liquid nitrogen temperatures for storage. However, the activity of the semen decreases considerably upon thawing; this decrease in activity is generally attributed to damage occurring to the semen cells during the thawing process. More specifically, a critical period appears to occur during the physical stage when the semen is in a semi-crystalline state, at the transition temperature range from a frozen storage state to a liquid operational state. It has been found that an increase in the rapidity with which the semen is passed through this critical semicrystalline state, results in a corresponding reduction in damage to the cells, and the resulting activity of the semen and success of the insemination is much greater than formerly observed.
In an effort to increase the activity of the thawed semen prior to injection, various expedients are disclosed in the prior art. For example, the semen is inserted into an elongated plastic tube or straw and the straw itself has been decreased in size and mass and designed to expose more surface area of the semen to the heating medium. Ice baths, as well as baths slightly higher in temperature than the ice baths, have been employed in an effort to increase the heat gain of the straw. However, these efforts have generally failed to produce an acceptable level of activity upon thawing of the frozen semen. In general, these prior art efforts allow the semen to remain in the critical temperature range of partially crystalline semen for an unacceptable length of time; and, as mentioned above, an undesirable amount of damage to the semen cells occurs.